1, Mechanisms underlying the chemoreception of Paramecium to quinine. By employing voltage clamp and ion substitution techniques, generation mechanisms of depolarizing and the hyperpolarizing quinine receptor potentials in Paramecium were examined. The depolarizing receptor potential is produced by activation of Ca channels while the hyperpolarizing receptor potential by K channels.b 2, Mechanisms underlying the chemoreception of Paramecium to alkaloids. Paramecium produced a depolarizing receptor potential in response to an application of chroloquine, strychnine or brucine at the anterior end of the cell, while a hyperpolarizing receptor potential to an application of these alkaloids at the posterior end. By examining the cross adaptation between these chemicals, it was found that these three alkaloids are received through the same pathway. The pathway is distinct from that for quinine. 3, Mechanisms of the chemoreception of Paramecium to the external ions.Membrane potential responses controlling the backward swimming of Paramecium in the K-rich solution were examined. The cell produced a sustained depolarization in response to an application of K-rich solution. The Ca conductance responsible for the backward swimming appeared as a prolongation of the depolarizing response. 4, Characteristics of membrane electric properties of Paramecium mutants with defects in chemoreception. Mechanisms underlying the prolonged backward swimming of Kag mutant of Paramecium to K-rich solution were examined. Membrane current responses corresponding to the action potential exhibited by Kag were almost identical to those exhibited by the wild type. However, inactivation time course of the K-dependent Ca conductance of Kag was10 times slower than that of the wild type. Kag shows prolonged backward swimming in the K-rich solution due to abnormal inactivation kinetics of the K-dependent Ca conductance.